Hitherto, there have been mainly produced two types of golf balls. The one is a solid golf ball, such as a two-piece golf ball, which is composed of a core formed from integrally molded rubber material and a thermoplastic resin cover (e.g. ionomer resin cover) formed on the core. The other is a thread wound golf ball which is composed of a solid or liquid center, a thread rubber layer formed by winding thread rubber around the center and a cover of ionomer resin or balata etc. having a thickness of 1 to 2 mm, covering on the thread rubber wound layer. The solid golf ball, when compared with the thread wound golf ball, has better durability and better flight performance because of larger initial velocity when hitting and longer flight distance. On the other hand, the solid golf ball exhibits hard shot feel when hitting, and shows difficulty to put spin on the ball, thus poor controllability at approach shot. The difficulty of putting spin on the ball comes from the structural features of the solid golf ball that an initial velocity is very high, when hitting, and a contact area of the ball with a hitting face of a golf club is very small.
In order to solve the problem, the cover is made from softer material to make the outer portion of the golf ball soft. However, the rebound characteristics of the golf ball are less than those of conventional cover and deteriorate flight distance inherent to the two-piece solid golf ball.
In order to solve the above problem, it is proposed that the cover is made two-layered, of which each layer is formed from a different material. For example, Japanese Patent Kokai Publication No. 244174/1992 proposes that a thermoplastic material comprising a block copolymer of amides is used in the inner cover layer and a thermoplastic resin is used in the outer cover layer. However, the resulting golf ball has poor rebound characteristics and reduces flight distance, because the block copolymer of amides has low rigidity and is used in the inner portion of the cover. Japanese Patent Kokai Publication No. 343718/1994 proposes the use of a hard high-acid content ionomer resin in an inner cover of a two-layer structured cover golf ball to increase the fight distance. The hard high acid content ionomer resin, which has high rigidity, is used in the inner cover, but the resulting golf ball has not hardness sufficient to increase a golf ball velocity because of softening the outer cover layer. It is required to make the inner cover layer harder to increase the ball velocity.